Bromine Incorporation Affects Phase Transformations and Thermal Stability of Lead Halide Perovskites

Mixed-cation and mixed-halide lead halide perovskites show great potential for their application in photovoltaics. Many of the high-performance compositions are made of cesium, formamidinium, lead, iodine, and bromine. However, incorporating bromine in iodine-rich compositions and its effects on the thermal stability of the perovskite structure has not been thoroughly studied. In this work, we study how replacing iodine with bromine in the state-of-the-art Cs0.17FA0.83PbI3 perovskite composition leads to different dynamics in the phase transformations as a function of temperature. Through a combination of structural characterization, cathodoluminescence mapping, X-ray photoelectron spectroscopy, and first-principles calculations, we reveal that the incorporation of bromine reduces the thermodynamic phase stability of the films and shifts the products of phase transformations. Our results suggest that bromine-driven vacancy formation during high temperature exposure leads to irreversible transformations into PbI2, whereas materials with only iodine go through transformations into hexagonal polytypes, such as the 4H-FAPbI3 phase. This work sheds light on the structural impacts of adding bromine on thermodynamic phase stability and provides new insights into the importance of understanding the complexity of phase transformations and secondary phases in mixed-cation and mixed-halide systems.

show that the same trends apply for Cs0.25FA0.75PbI3and Cs0.25FA0.75Pb(I0.83Br0.17)3as Cs0.17FA0.83PbI3and Cs0.17FA0.83Pb(I0.83Br0.17)3.Cs0.25FA0.75Pb(I0.83Br0.17)3has reduced thermal stability compared to Cs0.25FA0.75PbI3as evidenced by the appearance of the δ-CsPbI3 and 4H-FAPbI3 peaks appearing as low as 150 °C.The final phase transformation products of Cs0.25FA0.75Pb(I0.83Br0.17)3are dominated by PbI2, while without bromine, there is still substantial influence from the δ-CsPbI3 and 4H-FAPbI3 peaks.Spots 3 and 4 on CsFAPbI and CsFAPbIBr 250 °C are chosen to show that even though there are differences in intensity due to surface roughness, the relative composition of the emission is the same.A spot on the grain is compared to a spot on the deeper surface, and though the deeper surface is lower emission, the location of peaks and their intensities relative to one another are the same.I.e.comparing CsFAPbI 250 °C spots 3 and 4, 3 is lower intensity than 4 but both have a broad perovskite emission from β-perovskite and 4H-FAPbI3 with also some emission from PbI2.
Note S1: Threshold Determination for Pink PbI2 Overlay in Figure 2.
Intensity maps shown in Figure 2 were converted to black (no emission) and white (emission) using Adobe Illustrator.White areas were then extracted, and the black background removed before turning to pink and adding layer in overlay on top of SEM image.S3.DFT calculated lattice parameters and angles with changing composition.

Figure S10. XPS Fits for CsFAPbI Used to Calculate Ratios
Loss features were used to correct background but are not included in ratios.

Figure S11. XPS Fits for CsFAPbIBr Used to Calculate Ratios
Loss features were used to correct background but are not included in ratios.

Hysteresis Index
The hysteresis index is calculated using the difference between the power measured during forward and reverse J-V curves divided by the reverse.No hysteresis will have a value of 0, while maximum will have a value of 1. Hysteresis increases with increasing annealing temperature, as is expected due to the increased production of defects.However, hysteresis is a complex process that is affected by many factors other than defects, including morphology, grain boundaries, and inherent energetics.Thus, the results here cannot be attributed solely to changing vacancies with changing composition, as adding bromine also effects properties such as morphology as seen by CL-SEM (Figure 2).

IMPS Analysis
Low frequency IMPS measurements can be used to assess ionic movement by evaluating the changing peak position.A shift to lower frequencies indicates slower ionic transport.This is expected at higher annealing temperatures due to the likely production of defects (hypothesized to be halide vacancies).This decrease in speed of ionic transport at higher annealing temperatures supports the production of vacancies hypothesized by DFT and XPS.

Figure S3 .
Figure S3.Log scale XRD patterns of CsFAPbI and CsFAPbIBr annealed at 250 °C to show the presence of the β-perovskite peak even at these high temperatures, though the diffraction patterns are dominated by textured secondary phases.

Figure S5 .
Figure S5.Point Spectra Corresponding to Spots Indicated on Figure S4.

Figure
Figure S6.XRF Maps of Elemental Ratios for Varying Annealing Temperatures (CsFAPbI).

Figure S9 .
Figure S9.Evolution of perovskite surfaces.(a) I-only pristine surface.(b) I-only surface with V_I (iodine vacancy).(c) Mixed I-Br pristine surface.(d) Mixed I-Br with V_I.The formation of the vacancy induces a more significant increase in surface energy than the presence of Br alone, underscoring the substantial destabilizing effect of the vacancy on the perovskite surface.

Figure S14 .
Figure S14.Forward and Reverse Device Data for Both Compositions and Annealing Temperatures

Figure S7. XRF Maps of Elemental Ratios for Varying Annealing Temperatures (CsFAPbIBr) Table S2. Average elemental ratios for CsFAPbIBr XRF Maps (Figure S9)
Figure S8.Sample X-ray beam induced current mapping showing that areas of high Cs content correspond to low current.Areas with low Cs and high current indicate the presence of δ-CsPbI3.Scale bar is 5 μm.